Prior art rail vehicles sometimes have been provided with fluid pressure operated friction brake systems including some sort of an electrical assist feature. In a typical prior art system, a conduit known as a brake pipe extends from the operator's station aft through both the locomotive and any trailing cars. Also extending from the locomotive through the trailing cars is a main reservoir equalizing pipe which is pressurized to a predetermined level from a compressor located in the locomotive. Each individual rail car includes a control valve which responds, for example, to reductions in the brake pipe pressure by admitting pressurized fluid from a reservoir located in the rail car to the fluid pressure operated friction brake apparatus of the vehicle.
Where there are a large number of rail cars and a correspondingly long brake pipe running from car to car, significant delays in response can be experienced as a pressure signal initiated at the locomotive moves aft along the train through the brake pipe. Various efforts have been made to improve the speed with which the pneumatic signal moves through the train, such as the provision of mechanically and electronically actuated venting valves in the cars which modify brake pipe pressure locally in some manner to accelerate movement of the brake application signal through the train. To release the brakes in such prior art systems, it is necessary to repressurize or recharge the brake pipe so that the control valve will vent the pressure applying the friction brakes. As in the case of brake application, repressurization of the long brake pipe requires considerable time which may lead to an unacceptable delay in obtaining a full release of the friction brakes. Some prior art systems which deal with the problem of accelerating brake application and release by the use of electrically operated valving systems are shown in Instruction Pamphlet No. 78 of the New York Air Brake Company, published in 1970 for the PS-68 Brake Equipment. Also, U.S. Pat. Nos. 3,709,564 and 3,716,274 of the Westinghouse Air Brake Company deal with a related problem.
In the prior art brake systems of the type just mentioned and other brake systems not having electrical assist features, it is known to provide the individual rail cars with fluid pressure operated auxiliary equipment such as load levelling springs, pneumatically operated entrance and exit doors, fresh water supply systems and the like. Typically, such auxiliary devices are connected alternatively to the main reservoir pipe or the brake pipe of the vehicle. Thus, the auxiliaries will operate properly as long as pressure is maintained in the brake pipe and/or main reservoir pipe. However, in the event of a separation of one rail car from the train due to an accident such as a derailment, a situation may arise in which the auxiliary equipment, particularly the exit doors, is not able to function properly due to a shortage of pressurized fluid.
While the prior art does teach the use of electrically assisted fluid pressure operated brake systems, it has generally been the practice to design such systems with a large factor of safety so that the fluid pressure actuated brakes will function adequately even if the electrical assist feature is rendered inoperative. In recent years, however, more and more sophisticated brake systems have been developed which rely to a greater extent upon the accelerated brake application and release achieved due to the presence of electrical assist features. In order to assure proper train operation with the electrical assist features, it is mandatory that the electrical assist valves function properly on every trailer car. To ensure that these solenoid actuated valves are operative, the Federal Railway Association has required that a terminal test be performed on each train before it goes into service. To determine whether the valves in each car of a train have responded to an electric application and release, it is necessary to apply the brakes and then have an inspector walk the length of the train to determine whether the brakes on each car have been applied properly; and then to release the brakes and have the inspector again walk the length of the train to determine whether the brakes on each car have released properly.
In such a situation, several problems have found to be prevalent. For example, the solenoid actuated valves are only energized for a short period of time, even with a full service application. Thus, the inspector does not have sufficient time to walk the length of the train before the solenoids have become deenergized. Also, the electrical and pneumatic systems function in parallel, so that the presence of a brake cylinder pressure is not necessarily an indication that the electrical assist valves have functioned properly. It has been suggested that the electrical continuity checks could be made to establish that a circuit has been completed through the train; however, this does not provide a foolproof indication that the valve elements have actually moved in response to the electrical signal directed to the valve's solenoid. One prior art approach to monitoring such valve actuation is shown in U.S. Pat. No. 3,937,074.